For Immediate Release
Light, conductive and nearly as strong as steel, carbon nanotubes are
being combined with lightweight polymers to produce composite materials
with properties attractive for use on future space vehicles. But choosing
the right polymer for optimal mechanical performance at the nanometer
scale requires a lengthy trial-and-error process.
By adapting the tiny cantilever and position measurement systems used
in atomic force microscopy (AFM), researchers at the Georgia Institute
of Technology are helping their NASA colleagues shorten that process.
Using chemical force microscopy, they are producing detailed information
about adhesion between single-walled carbon nanotubes (SWNTs) and molecules
of candidate polymers with different functional groups.
"Our hypothesis is that the stronger the adhesive interaction between
molecules and nanotubes, the more likely it is that the polymer will fully
wet the nanotubes, break up aggregations of nanotubes and form a mechanically-sound
composite," said Larry
Bottomley, a professor in the Georgia Tech School of Chemistry
and Biochemistry. "The intent is to come up with two or three
chemical groups that will give us the strongest interaction, and then
incorporate these onto polymers for further studies."
Details of the research were presented March 23 at the 225th American
Chemical Society National Meeting in New Orleans, LA. The Advanced Materials
and Processing Branch of NASA's Langley
Research Center has supported the work under grant NGT-1-02002.
In a traditional AFM, a gold-coated tip just 20 to 50 nanometers in diameter
is placed on the end of a tiny cantilever beam 200 microns long and 40
microns wide. The tip is then lowered onto the surface, which is then
moved under the probe.
Molecular-scale elevations in the surface cause small deflections in
the cantilever as the surface moves beneath the tip. A laser beam is reflected
off the backside of the cantilever onto a position sensitive detector.
The voltage from the detector is proportional to the deflection of the
cantilever. A computer is used to transform the resulting data into a
three-dimensional image of the surface.
Instead of mapping a surface, however, the Georgia Tech researchers use
the cantilever beam and deflection measurement to study the adhesion force
between alkanethiol molecules on the tip and nanotubes on the surface.
The researchers raise a surface composed of nanotube bundles until it
contacts the tip. When the nanotubes on the surface contact the alkanethiols
on the tip, they adhere to it. When the surface is lowered, the adhesive
force between nanotubes and polymer pulls the cantilever down.
"If there are no adhesive interactions between the tip and the sample
surface, the cantilever tip just lets go cleanly when you lower the surface,"
Bottomley explained. "If there is strong adhesive interaction, the
adhesive interaction bends the cantilever down until the restoration force
of the cantilever exceeds the adhesive force. That provides a direct measurement
of the adhesion."
The adhesion forces they are measuring with this method are in the nano-Newton
From that information, Bottomley and collaborators Mark A. Poggi of Georgia
Tech and Peter T. Lillehei of NASA can judge which polymers and
functional groups provide the best adhesion to the nanotubes.
To properly interpret the data, the researchers must know how the surfaces
interact mechanically. For instance, if the tip containing the polymer
touches ridges of a nanotube bundle, the adhesion will be less than if
the tip contacts a valley in the bundle.
"There is a very strong dependence on the sample topography and
the adhesive interactions we measure. Knowing the shape of the tip and
knowing where on this surface to find ridge lines, we can extract out
the adhesive interaction between specific functional groups on the tip
and the nanotube surface," Bottomley explained. "The broadest
impact of this work may be on other people doing this type of molecular
study using surface force apparatus or atomic force microscopy. They must
take into consideration the area of contact."
Instead of a three-dimensional map of the surface, the technique produces
a force volume image showing adhesion force variations across a two-dimensional
"We find dramatic differences in the adhesive interactions with
subtle changes in the chemistry of the tip," Bottomley said. "You
have the strongest interactions in the amine-terminated samples compared
to the methyl-terminated, hydroxyl-terminated and carboxyl-acid-terminated
Developed a decade ago, carbon nanotubes possess many attractive properties.
But they also tend to clump together into bundles, which can pose problems
in composite manufacture. If the polymer does not interact with or "wet"
the nanotubes individually, the result is a mechanical defect that will
weaken the resulting composite.
"If the polymer doesn't wet the nanotubes properly or if the nanotubes
aggregate, you get a composite in which portions are just the standard
polymer," Bottomley explained. "The real challenge is distributing
the nanotubes throughout the polymer in a proper orientation."
For the future, the researchers plan to test additional polymers and
functional groups, and to study the interaction of single nanotubes with
the polymer molecules.
A paper describing the work has been submitted to the journal Nano
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